Polyester polyols are hydroxy-functional condensation polymers made by reacting aliphatic or aromatic di- or polycarboxylic acids with polyols (usually diols). Polyester polyols react with polyisocyanates, chain extenders, and other components to produce polyurethanes for foams, coatings, adhesives, sealants, elastomers, and other applications. For example, polyester polyols based on aliphatic diacids are commonly used to make polyurethane elastomers for shoe soles.
Polyester polyols based on aromatic diacids and their derivatives, most notably terephthalic acid, isophthalic acid, and phthalic anhydride are well known. Aromatic diacids impart mechanical strength, thermal stability, chemical resistance, and other attributes to products made from their polyester polyols. Some aromatic polyester polyols are particularly valuable for making, e.g., flexible packaging adhesives or rigid foams for insulation or automotive instrument panels. For many polyester polyol applications, phthalic anhydride is the aromatic “diacid” of choice because of its low cost, ease of formulation, and low tendency to have precipitates compared with either terephthalic acid or isophthalic acid.
For end-use applications in which there may be indirect contact of the polyurethane with food (e.g., an adhesive used to bond layers of packaging film), formulators and regulators have redoubled efforts to identify and limit or eliminate traces of by-products having a tendency to migrate. For instance, by-products that lack hydroxyl functionality but are present in a polyester polyol (e.g., a cyclic ester) will not react with polyisocyanates to form a high polymer. Consequently, these by-products could migrate from the ultimate polyurethane adhesive.
In o-phthalate-based polyester polyols, cyclic esters are potentially generated by self-condensation of an o-phthalate monoester. The tendency to cyclize should depend on the nature of the diol reactant (chain length, branching, and other factors), but the degree to which such cyclic esters will form in a process designed to make the polyester is generally not well understood. Numerous references (e.g., U.S. Pat. No. 6,569,352) teach the preparation of phthalic anhydride-based polyester polyols, but these references do not recognize the issue of cyclic ester formation arising from the use of certain diols. Thus, for instance, DEG and 1,4-butanediol are typically taught as the equivalent of EG.
Ehrhart (J. Org. Chem. 33 (1968) 2930) showed that macrocyclic o-phthalate esters can be generated by thermolyzing the corresponding polyester polyol. In particular, the o-phthalate polyesters of diethylene, triethylene, 1,5-pentylene, and 1,6-hexylene glycols can be converted nearly quantitatively to the respective macrocycle, while other glycols, including ethylene glycol, give poorer yields of the macrocycle. The amount of cyclic ester present in the polyester polyol prior to thermolysis was not determined. U.S. Pat. No. 2,092,031 also reports making the macrocycle from phthalic anhydride and ethylene glycol by depolymerizing the corresponding polyester polyol (see Table 1).
U.S. Pat. No. 6,515,080 teaches a process for making polyethylene terephthalate modified with o-phthalate units. A pre-condensate of phthalic anhydride and ethylene glycol is made using a 2-3.5 molar excess of ethylene glycol. As shown in Table 1 of the '080 patent, condensation polymerization of phthalic anhydride and ethylene glycol can produce about 3 wt. % of the corresponding cyclic ester (mol. wt.=192 g/mol), although this cyclic ester was apparently not seen in a similar experiment (Table 2).
A variety of polyester polyols based on phthalic anhydride and diethylene glycol (DEG) are valuable commercial products for urethane coatings, adhesives, sealants, and elastomers. Examples include Stepanpol® PS and PD series polyols such as Stepanpol PS-20-200A, PS-1752, PD-110 LV, PD-200 LV, and PD-56. Despite its utility in polyester polyols, DEG may eventually become scarce because of a trend in the chemical industry favoring production of “ethylene glycol only” instead of its mixture with DEG, triethylene glycol, and higher glycols.
Our own recent work, outlined below, indicates that cyclic esters (typically about 3-8%) are produced in the production of polyester polyols, particularly when DEG, 1,4-butanediol, and other common glycols are reacted with phthalic anhydride. However, for reasons discussed earlier, it is often desirable to make polyester polyols, particularly o-phthalate polyester polyols intended for CASE applications, with very low levels of cyclic esters.